In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. In addition, it may be critical to acquire optimal wellbore placement to enhance maximum oil production. An azimuthal directional electromagnetic (EM) resistivity tool has been implemented to actively adjust borehole position so that an optimized well plan can be achieved. A related application of such a tool is geosteering, which is an intentional control to adjust drilling direction. A geosignal is a signal or representation that can be used for geosteering. Azimuthal geosignal responses can be used to guide well path orientation in real-time as well as steer the well towards a desired formation zone It is widely known that existing azimuthal EM tools used in geosteering suffer a “blind-spot” problem in a drilling condition where the logging tool is located at layered formation media with symmetric resistivity profiles. In such circumstances, geosignal responses of current (conventional) directional EM tools become very insensitive to such layered formation effects so that petrophysicists may misinterpret the formation geology based on conventional geosignal signals. An example of an extremely difficult formation case includes a tool located in the middle layer of a symmetric 3-layer formation resistivity profile, where the middle layer has a higher resistivity value than both the upper and the lower layers. In this drilling condition, the geosignal is very weak and petrophysicists may make the wrong impression that the tool is drilling in a homogenous formation medium.
WO 2011/129828 A1 discusses various embodiments that include apparatus and methods of processing and geosteering with respect to well logging. Methods and associated apparatus can include acquiring signals generated from operating a tool rotating in a borehole of a well, where the tool includes a receiver antenna tilted with respect to the longitudinal axis of the tool and two transmitter antennas. The acquired signals can be processed with respect to a direction in the rotation of the tool to determine properties associated with a formation and/or to determine a geosignal for geosteering a drilling operation. WO 2011/129828 A1 includes discussion of converting acquired signals to coupling components.
US Patent Publication 2008/0078580 relates to systems and methods for performing bed boundary detection and azimuthal resistivity logging with a single tool. Some method embodiments include logging a borehole with an azimuthally-sensitive resistivity logging tool; deriving both a resistivity log and a boundary detection signal from measurements provided by said tool; and displaying at least one of the boundary detection signal and the resistivity log. The resistivity log measurements may be compensated logs, i.e., logs derived from measurements by one or more symmetric transmitter-receiver arrangements. Though symmetric arrangements can also serve as the basis for the boundary detection signal, a greater depth of investigation can be obtained with an asymmetric arrangement. Hence the boundary detection signal may be uncompensated.